Recent publicity arising from a study by the Universities of Southampton and Tasmania
[1] of an old
sea level benchmark at Port Arthur, Tasmania, claims `dramatic' sea level rise of about 13cm since the mark was first struck back in 1841.

Fig.1 - The benchmark at mean tide

These claims are based primarily on statistical probability models and are unsupportable when viewed in the context of
other physical and documentary evidence available surrounding the benchmark.
The study claims the benchmark was originally set 44.5cm above the mean level of the sea
(as it then existed in 1841). Since it now sits at 31.5 cm above
(see fig.1), the difference - 13cm - is claimed to be evidence of sea level rise.

However, the man responsible for putting the mark there, explorer Sir James Clark Ross stated explicitly and

several times in his 1846 book [3] that the mark was placed at MSL (as he estimated it to be), not at a point 44.5cm above,
near the high tide point, as claimed by the study.
Other evidence surrounding the original placing of the benchmark is less clear, but we do have one positive measurement of where the benchmark stood relative to sea level taken in 1888 by the then Government meteorologist, Commander J. Shortt R.N.
He found the mark to be 34cm above sea level - only 2½ cm different to its current
position [7] [5].

This suggests a sea level rise since 1888 of only 2½cm, not 13cm as claimed by the study.
This small rise of 2½ cm is fully consistent with a survey of long-term tide gauges
[15] around the Australian coast carried out recently by the National Tidal Facility in Adelaide, which found a sea level rise rate of only 0.3 mm/yr, equivalent to a sea level rise of 3cm over a century. Even the current sea level in the inner cove
(Mason Cove) of Port Arthur itself is lower than that indicated by a tide gauge
[6] which the study claims existed there in the early 1840s.

The 1888 measurement by Cmdr. Shortt suggests sea levels rose an incredible 44.5 - 34 = 10½ cm in only 47 years, at a time of global cooling
and slowly uplifting land when quite the opposite should occur. While the study dismissed
Cmdr. Shortt's measurement as inaccurate, with no coherent explanation given for that
view, it is much more probable that the study simply got the original benchmark height wrong and that the 1888 measurement by Cmdr. Shortt was sound.

Complicating the issue further is the possibility, expressed by Cmdr. Shortt himself, that numerous earth tremors in
from 1883 to 1886, unprecedented either before or since, may have uplifted the land
by an unknown degree over that period of time. Indeed, this was his stated reason for making the measurement in the first place, having received reports of just such an effect from islands in the Bass Strait.
Any land uplift from the 1880s tremors would displace the benchmark upwards from its original position if the benchmark had indeed been placed at
MSL as stated clearly by James Clark Ross.

In addition, modern tide gauges located at Hobart and Spring Bay on either side of the Port Arthur site, on the same coast, show no sea level rise since their installation during the
1980s [18].

The weight of physical and documentary evidence shows that sea level rise around Tasmania, and indeed Australia as a whole, is only very slight, and that this study is seriously in error. The reason may well be the over-reliance on statistical probability modelling at the expense of
physical and documentary evidence.

Introduction

This report has been prompted by the recent publication of
a paper by Pugh et al [1], giving details of a 7-year study by the
Universities of Southampton and Tasmania into the origins, history, and current
height of a sea level benchmark etched onto a cliff on a small island in Port
Arthur Harbour,
Tasmania, in 1841.

The authors claim the benchmark was not struck at
Mean Sea Level (MSL) as described by the man responsible for putting it there, Antarctic
explorer Captain Sir James Clark Ross, acting in association
with Thomas Lempriere (a senior official of the Port
Arthur convict colony), but was rather struck closer to high tide, the mark estimated by Pugh et al. to be 44.5 cm above MSL as it existed in
1841. Since they report that the mark is now 31.5 cm above current
MSL, as determined from modern tide gauge data, they propose
that a sea level rise of 44.5 - 31.5 cm = 13 cm has therefore occurred since
1841. Of this, they further propose that the current land uplift
rate would raise the land 3.2 cm over the same period, making a net sea
level rise of 13 - 3.2 = 9.8 cm.

There are a number of height positions relative to MSL in 1841 to which the benchmark might have been struck - at
44.5 cm
above as claimed by Pugh et al., or 14 cm above as suggested later in this
report or at the actual 1841 MSL itself. Any such scenarios involve
unresolved issues with available evidence, the most serious of which is
the Pugh et al scenario which completely contradicts the written account given by
Ross in his 1847 book [3].

Between 1841 and the modern era, we have only one
measurement made of the height of the benchmark relative to sea level, and
that was organised by Commander J. Shortt R.N., the Government Meteorologist of
the day, on 24th February 1888. The height he reported is only 2½
cm different to the height today, a sea level rise of only an inch in 114
years.

It is the Shortt measurement of 1888 which
is critical for any scenario. In the case of Pugh et al., they have applied a
very long error bar to the Shortt measurement, 26 cm, or about one third
of the full tidal range at Port Arthur. It is only through this wide
error bar that the scenario favoured by Pugh et al works at all as the sea
level rise they propose depends on their sea level trend line passing through the
extreme lower end of their 1888 error bar. A narrower error bar would
require a sea level rise between 1841 and 1888 of around 10 - 12 cm, much
too large for only 47 years during a period of global cooling and against
the trend of a gently uplifting coast.

This report demonstrates some key problems with the Pugh et al. study and to provide an alternative scenario in
Part 2 consistent with the known evidence surrounding the Ross-Lempriere
benchmark.

The Ross-Lempriere Sea Level
Benchmark Today

Fig.2 - SE Tasmania

We must first establish some geographical
context. Firstly the general location -

Port Arthur is a large harbour in south-eastern Tasmania, well sheltered from the
ocean, and opens out directly to the Southern Ocean with no
large rivers entering it. The harbour area is largely
undeveloped and was the site of a British convict settlement
during the middle of the 19th century, the settlement being also
called Port Arthur. There are the ruined remains of the once-busy 19th century convict
colony, a popular spot today for visitors, and a small settlement of a few dozen houses on the southern shore
of the harbour. Everywhere else is mostly forest and undeveloped shoreline.

Port Arthur sits on a large peninsula, actually two of them
linked in series.

Fig 3. - The Tasman Peninsula

This allowed the convict guards in the 19th
century to prevent escaping prisoners from getting past a narrow
isthmus of land at Eaglehawk Neck (top of
fig.3). Eaglehawk Neck is also of interest to the
overall story as it is only 106 metres wide [12], wedged between the
Southern Ocean to the east and Eaglehawk Bay to the west (which also opens out to the ocean).

Port Arthur's harbour mouth faces south and opens out directly to the Southern
Ocean. Consequently, it experiences tides unimpeded by large land masses or
estuary effects. For this reason the tidal range at Port
Arthur is quite small, around 75cm, (or 2½
feet) between high and low tides on
average.

The tides are complex however, having alternate minor and major
tides, plus two seiches, or water swills, within the harbour, one
with a cycle of about an hour, the other with a cycle of less than
a minute. The one-minute seiche is quite visible to the eye.

Fig. 4 - Map of Port Arthur

The
old convict colony is now a managed Historic Site and comprises the main convict settlement at Port Arthur, a smaller ruin settlement for convict boys at Point Puer, and finally, the
`Isle of the Dead', the final resting place of nearly 2,000 people, convicts, children, and free persons, buried there during the convict days of early settlement last century.
The Isle of the Dead lies about a kilometre east of Commandants Point and it
is here that the 1841 sea level benchmark sits on the north side of the
isle.

Four locations from the above map are of special
interest for our purposes -

1) The Isle of the Dead. The Ross-Lempriere
benchmark is located on the north side of this island.

2) The Basin and Stores area. This is where Pugh et al. believe a tide gauge was located in 1841.

The sea level benchmark engraved there on the north
side of the island (facing away from the photo,
fig.5)
was originally cut on 1st July 1841, by a senior official of the penal
colony, Thomas Lempriere, in association with Captain Sir James Clark
Ross, the renowned Antarctic explorer, who was visiting Tasmania at the time. Ross
was the commander of a scientific expedition to the Antarctic, using two
ships, Erebus and Terror, both of which were moored at
Hobart between April and July of 1841 during a three-month stopover in
Tasmania.

It was Ross who set
in motion the project to establish a sea level benchmark at Port Arthur,
a project fully supported by both the Governor of Tasmania,
Sir John Franklin, and the British Colonial Office. In other
words, this project was conceived at the highest level.

The benchmark they established is still there and in perfect condition -

The mark itself currently sits 31.5 cm above
MSL, as
measured by Pugh et al. Both the MSL at Port Arthur and the height
of the benchmark above it were calculated by them as a result of
extensive survey work involving levelling and altimetry
satellites. The figure they give is consistent with other
estimates given of the current height of the benchmark and is not itself
in dispute here.

Pugh et al. also estimate that gradual land uplift since 1841
was 3.2 cm, which, when deducted from their estimate of sea level
rise, gives a net sea level rise of 9.8 cm.

Sea Level Rise?

This estimate of sea level rise made by Pugh et al.
depends critically upon exactly what height relative to MSL (as it existed in 1841) the benchmark was
originally struck to by Ross and Lempriere. It also depends
critically upon a height measurement made of the benchmark by Commander
J. Shortt R.N., the government meteorologist, 47 years later in
1888. As we shall later see, it is the 1888 measurement by Cmdr.
Shortt which is the weakest link in the Pugh et al. scenario of sea
level rise.

The above diagram is reproduced from a similar diagram in the Pugh
et al. paper,
but drawn to scale marks to show the levels, error bars and years more clearly.
It shows the scenario they present of sea level rise. The Ross-Lempriere
benchmark is taken to lie at the zero line at the top of the graph, and the changing
sea levels are referenced against it. As the diagram shows, they
assume the sea level in 1841 to be at -44.5 cm below the benchmark, thus giving a relative sea
level rise to the present of 13 cm.

As the chart indicates, the scenario of sea level rise
depends - (a) on the choice of 44.5 cm as the starting point above MSL in
1841, and (b) on the large error bar of the 1888 Shortt measurement, the
extreme bottom of which is needed to allow the rising sea level trend line to
pass through to the present. Indeed, at a lecture given by the study
authors to explain
the results [2], it was pointed out that because
there was no evidence from elsewhere in the world that sea levels rose in
the mid to late 19th century, then the sea level line from 1841 should
really run horizontally to 1890 and then curve sharply upward from there to
the present. In light of this, we can modify the timeline graph accordingly -

If that is the case, then the line would have to run
horizontally from the upper extreme of the 1841 error bar though the extreme lower tip of the 1888 Shortt
error bar. The slightest need to narrow or even raise the Shortt
error bar would put the `sea level rise' scenario in considerable
difficulty and also call into question the validity of their 1841
benchmark height placement. Their scenario cannot accommodate any
land uplift pre-1888 as that would impose a slight sea level fall to 1888,
again putting their trend line out of reach of the Shortt error bar.

The Recent Past

The authors were very mindful of the claims of global sea level rise
made by the
U.N. Intergovernmental Panel on Climate Change (IPCC).
It claims a 10 to 20 cm sea level rise since 1900, a point of some
controversy, but which
Pugh et al. regard as consistent with their result of 13 cm relative sea
level rise at Port Arthur.

Firstly, can we see recent sea level rise? For Port Arthur, we
simply don't know because the CSIRO tide gauge has been operating there
only a few years, and mean sea level determination needs 18½ years of
data to get a fully accurate figure. This is because it takes that
long for all the variations and permutations of lunar and solar cycles
to work themselves through one full cycle. However, there are
longer-term tide gauges located at Hobart to the northwest and Spring
Bay to the north. At the lecture it was stated - "We know that sea
level can't change very much between Hobart and Port Arthur. Simple
calculations show that it only varies at most 1 or 2 centimetres."[2]. It is this close linkage between Hobart
and Port Arthur tides and sea levels which has enabled Pugh et al. to
give a figure for current MSL at Port Arthur as being 31.5 cm below the
current height of the benchmark. This linkage between
Hobart and Port Arthur makes sense because both harbours are adjacent to
each other and both face south, opening out directly to the Southern Ocean. Spring Bay to the north
is different in that it sits on the east coast of Tasmania with a sea access
facing east toward the Tasman Sea.

Here are the sea level histories for Hobart and Spring Bay, plotted from
data published online by the Permanent Service for Mean Sea Level (PSMSL). [18]

Fig.9 - Monthly Relative Sea Level at Hobart and Spring Bay [18]

Hobart has the slightly longer record of the two and
neither shows any general tendency for sea level rise - a lot of variability certainly,
but no significant upward trend. The lowest sea level recorded at both sites
was quite recently - late 1997, right in the middle of a major El Niño
event, a separate issue which will be returned to later.

From this data, we can confidently conclude that whatever sea level
rise may have happened since 1841, none of it happened since 1987 when
the Hobart tide gauge data from PSMSL begins. If Pugh et al. were right, then
the sea level rise they claim - all of it - must have been pre-1987.

This brings us to the work of Bruce Hamon[5], who researched the benchmark well before
Pugh et al., publishing a paper on the subject in 1985.
Hamon also published his paper well before climate change and sea level
rise became a political issue and his paper can therefore be viewed as a fairly
neutral assessment on his part. Using the
Hobart tide constants as his guide, he determined MSL at Port Arthur to
be 36 cm below the level of the benchmark, although he himself put an
error range of ±5 cm for any single spot observation - such as the one
he himself made. This makes his observation and associated error
bar within the latest
estimate of 31.5 cm given in the Pugh et al. study.

But Hamon did more than merely measure the benchmark relative to MSL
in 1985. He proceeded to use Hobart tidal constants to effectively
go back in time. Just as tides can be predicted years into the
future, once each areas tide constants are known, it is equally possible to
predict tides years into the past, many decades even. Hamon took
his analysis right back to 1888 when Cmdr. Shortt made his measurement
of the benchmark.

The 1888 Height Measurement

Commander J. Shortt R.N. was the government meteorologist of the
day. He published government notices regularly in the local press
and was a contributor of scientific papers to the Royal Society of
Tasmania, the state's premier scientific body.

During the early 1880s, and immediately in the
wake of the mammoth eruption of Krakatau in August 1883, Tasmania was subjected to over a thousand earth
tremors, mostly on the eastern half of the island [9], and Shortt published an analysis of these in 1885 [8]. These
tremors were unprecedented in number or scale either before or
since. Three years later, Shortt published his paper on the Ross-Lempriere
benchmark [7], and explained that it was because of the possible vertical
movement of land following such tremors that he decided to measure the
height of the benchmark so that future vertical movements of the land,
if any, could be accurately tracked.

Shortt himself did not do the actual measurement. He
requested a
local official, `Mr. T. Mason' to do it for him(the same
name as the inner cove of the Port Arthur convict settlement
- Mason Cove.) According to Shortt -

"Mr. Mason, at my request, very kindly ascertained
the time of low water on February 24th, 1888, at 11 h. 45 m. a.m., which
day corresponds relatively with the moon's age 45 years previously (the day the benchmark was originally struck -
1st July 1841 - JD). At this low water level the mark was found to
be 2½ ft. above. This very closely corresponds with the normal
difference between these levels of low and high water,...".

Clearly this is a `spot' measurement
and potentially contains a lot of room for error, amply justifying a
large error bar on the timeline graph in the absence of any other
information which might narrow down those errors. There are
potential errors from the observer himself, there are errors arising
from the complex pattern of tides in that region, and there are errors
which might arise from meteorological variability such as strong
westerly winds making the tides pile up, or atmospheric pressure changes
which can raise or depress the local sea level. Any or all of
these uncertainty factors calls for a significant 1888 error bar on the
timeline graph.

With regard to observer
error, if the observer was an outsider visiting Port Arthur for the
first time, then such an observer would have no idea if the measurement
they were making was normal for that location or somehow anomalous due
to some local variable skewing the sea level on that occasion. The error
bar for such a measurement would have to be very wide in the absence of
any application of knowledge about local tides, winds, or pressure.

Then we have
what we might call an `informed spot measurement', one where the
observer already has some of that additional information and acts on it.

My photograph of the benchmark (fig 6)
represents just one such `informed spot measurement'. In my first
report over two years ago on this issue, I estimated the current height
of the benchmark to be 30 cm above mean sea level. Since we now
know it is 31.5 cm above, that was an error of only 1½ cm on my
part. How did I manage to achieve such close accuracy?
It could of course be plain luck, but I did choose a day close to
the same age of the moon as on the day
the benchmark was struck. Next, I checked the
National Tidal Facility in Adelaide to find out the times of the tides
at nearby Hobart and Spring Bay on the day I intended to visit the Isle of the
Dead. From this, I determined the most likely time of mean tide (not itself mean sea level, but close). I even checked the barometric pressure
on the day to make due allowance for that also.

With that information, I hired a boat
and took the photo of the benchmark at the expected time of mean tide. Once there, I found a lot of swell
in the vicinity of the cliff where the benchmark was located, but from
carefully observing the ebb and flow of that swell, I was soon able to settle
visually on where the mean level at any one time was. I took the photo
at the time of expected mean tide.

When I looked at the photo afterward,
I saw that the benchmark was sitting about 40 cm above the water (using the horizontal line of the benchmark -
50 cm - as a guide. See fig.6). Since
atmospheric pressure was 1024 mb on the day I did it, thus depressing
sea level by nearly 10 cm, I subtracted that 10 cm from the 40 cm visual
height to arrive at a figure of 30 cm for the adjusted height.
Nothing too difficult or complicated. No ruler or measuring
stick, just knowledge of the tide times, moon phase, and the atmospheric
pressure and wind.

My final answer, 30 cm, ended up only 1½ cm different to the real figure
determined at some considerable cost of personnel time and a
state-of-the-art tide gauge. If I could do that, it is simply not
credible to imagine that someone else with better local knowledge than
mine would
end up incurring an error bar 26 cm wide.

This example is given to show that
the claimed difficulties in making a good spot measurement may be
over-rated, perhaps for reasons of professional pride rather than any
genuine difficulty, and that it is not necessarily inaccurate provided it is integrated
with that additional information such as knowledge of tide times,
heights, meteorology, and phases of the moon. If I could do it to within 1½
cm, with such simple methodology, there's no reason to suppose a local
man with local knowledge, Mr. Mason, could not do
equally well or better, especially as he was measuring it from the low
tide point and not the more turbulent mean tide that I used. A 5 cm error bar,
as suggested by Hamon, to allow for potential observer error is therefore
perfectly reasonable in the circumstances.

Tidal variability is
sufficiently complex at Port Arthur to impose a significant error on
Shortt's measurement. However, this was removed by Bruce Hamon and
his backward projection of tides. Because Shortt gave the exact
date and time of the measurement, 11.45 a.m. on 24th February 1888,
Hamon was able to determine where low tide should have been on that day,
based on the Hobart tide constants. From that he was able to
correct for any error created by tidal variability. He wrote -

"Shortt found the mark 2ft 6in
(0.76m) above low water, at 11.45 local time on 24 February 1888.
Again using the Hobart tide constants, I found the tide at this time to
be 0.42 m below mean sea level. Predicted low tide was about 30
minutes later, and only 1 cm lower. "

So, the observer error range is about 5 cm, and tidal error has been largely removed by Hamon's
calculation. That leaves the meteorological error. This
is potentially quite large because strong westerly winds in southeastern
Tasmania cause a `pile up' of the tides and result in very high tides
all along the east and south coasts, including Port Arthur. In addition,
every millibar of pressure above or below the mean pressure of 1014.8 mb [5]
results in a raising or lowering of sea level by about 1 cm. That
imposes a huge uncertainty on any spot measurement since it could make
the sea level as much as 10 cm higher or lower than normal. It
certainly did so on the day I photographed it.

On this issue, Hamon regrets to inform
us - "We cannot apply a
correction for atmospheric pressure in this case, as no Hobart data for
for 1888 are available."
That lack of atmospheric pressure and wind data means that the Shortt
observation would need a wide error bar after all.

During a recent discussion group emailing
over a period of a week, after receipt of their paper and attendance at
their lecture, I asked one of the authors what the basis was for the long error bar
applied to the Shortt/Hamon measurement for 1888, as the paper itself
was somewhat vague on the point. The reply -

"After adjusting for the tides, you are still left with the meteorological variability, which is simply caused by the effects of the weather (e.g. winds and atmospheric pressure) on the oceans." [pers.
comm.]

I took this to mean that the big
unknown
that Hamon acknowledged, atmospheric pressure, formed the basis
for the large error bar applied to the Shortt/Hamon observation, its
size making it possible for the sea level timeline in their graphic (fig.8)
to squeeze in through the extreme lower tip of the Shortt error bar.

Although Hamon did not have
those atmospheric pressure or wind figures, they were on public record.
Once I concluded from the email discussion that pressure and wind
uncertainty was the primary reason for the long 1888 error bar used by Pugh et al.,
it took me about half an hour at the public library to find the relevant
data on pressure and wind in 1888.

On 24th February 1888, the atmospheric
pressure at Hobart was 29.96 inches, which converts to 1014.7
millibars. The wind was light south-easterly [10].
This information was contained in the `Shipping News' of one of the
local papers of the time, the Daily Telegraph. Pugh et als.
determination of such a large error bar for
the Shortt measurement suggests they were including meteorological
uncertainty as a key factor in their calculation of that error bar, an uncertainty which
now no longer exists.

Now that we have the wind and
pressure data, that particular uncertainty is redundant. Since
Hamon estimated modern mean pressure in the region to be 1014.8 millibars, the lack of
wind and the Hobart
pressure for that day, 1014.7 mb, means that no adjustment is
necessary. The figure of 34 cm can stand without any significant correction or
uncertainty allowance for wind or pressure. A light south-easterly
wind on 28th February would have minimal effect on the height of the
tide that day. The pressure of that day, 1014.7 mb, is only
0.1 mb different to the modern mean pressure for that area reported by
Hamon. Consequently, the sea levels measured in 1888 and in 1985
by Hamon are directly comparable since the two atmospheric pressures
were practically the same.

Clearly, an error bar 26 cm long on the
Shortt measurement, fully one third of the tidal range, is at odds with observation now that both
tides, pressure, and wind have been accounted for. Only a smaller
error bar to cover observer error and other minor variables can be entertained. Hamon
suggests ±5 cm for such spot observations when done carefully, and that
is the revised error bar we should now apply -

There is one further issue with the
Shortt observation of 1888. In that year, a full El Niño was in
progress, a particularly severe one causing widespread drought and
bushfires throughout Tasmania, and widely reported in the papers of the
time. The key measure of El Niño, the Southern Oscillation Index(SOI) averaged below -10 for 1888, a
particularly strong event [20].

This has consequences for the Shortt
measurement.

An El Niño has the effect of temporarily depressing
mean sea levels throughout the southwest Pacific, including Tasmania,
such as shows up clearly on this sea level record from Devonport
on the north coast of Tasmania, where sea level appears to drop
about 10 cm for the duration of the event. Data from PSMSL.

The
recent strong El Niño in 1997-98 also had a similar effect on the tide data
for Hobart and Spring Bay (fig.9) forcing sea levels to their lowest point in
the record.

Fig.11 (left)
- Monthly sea levels for Devonport, Tasmania

This means, the length of the Shortt
error bar should not only be shortened to ±5 cm as shown earlier, but
also be shifted upwards in fig.10 by an amount sufficient to compensate for the El Niño
and associated lowered sea level. One of the authors said this in
a subsequent email discussion on this particular issue - [pers.comm.]]

"From a comparison of Spring Bay sea level data and SOI, I find that sea level varies approximately
as SOI/40 m. Now SOI for February and March 1888 was -0.33 and -1.47 (see
http://www.cru.uea.ac.uk/ftpdata/soi.dat
). For an approximate SOI value for 24 Feb. 1888, I therefore take an average of -0.33 and
-1.47, which is -0.9. The average SOI for 1866 to 1997 is -0.067,
which I will take as a reference value. Therefore the SOI adjustment should be (0.9-0.067)/40 = 0.02 m."

That's a 2 centimetre error factor which should have been
integrated into
the Shortt error bar. I checked the Spring Bay data from PSMSL for myself. Instead of using
a SOI-based procedure as quoted above, I averaged the whole data from 1987 to 2000, then subtracted from
it the
average of the months of the 1997-98 El Niño. Sure enough, the
answer also came to 2 cm. even though my method was
different. Since the SOI is a statistical index based on
comparative air pressure between Darwin and Tahiti, I did not regard it's use as a constant in
local sea level
determination to be particularly appropriate, as subtracting averages of
sea levels directly made much better sense. Besides, using the
pressure-based SOI as part of a sea level calculation creates an
`apples and oranges' effect which may look mathematically elegant, but
in reality be somewhat meaningless. That we came to the same answer
for Spring Bay is interesting, perhaps coincidental, but would not
necessarily agree for other stations.

However, both Hamon and Pugh et al. have consistently used Hobart as
their frame of reference for Port Arthur, particularly as both harbours
face south, whereas Spring Bay faces east and more easily subject to
wind and coastal effects on the tides. Performing the same
subtraction of averages on the Hobart data yielded an El Niño sea level
effect of 5 cm, not 2 cm. The statement at the lecture
- "We know that sea
level can't change very much between Hobart and Port Arthur. Single
calculations show that it only varies at most 1 or 2 centimetres." means
that any El Niño effects at Hobart will show up with a similar
magnitude at Port Arthur. It is this 5 cm correction which should
therefore be applied to the Shortt measurement of 1888, not the 2 cm one
would get from Spring Bay. The chart now looks like this -

At this point, the `sea level rising' scenario
offered by Pugh et al. is in deep difficulty. With a narrower error
bar for Shortt, and its
upward shifting to account for the El Niño of 1888, the bulk of their sea level rise of 13 cm has to now be
accomplished between the years 1841 and 1888, a period where the authors
themselves acknowledged in the lecture saw no general sea level rise at all anywhere else in the world.
The Shortt measurement is now scarcely different to the current position of the benchmark. The idea
that a steep sea level rise took place between 1890 and 2000 is
simply not credible. In addition, land uplift of about 2½
cm since 1888 would make the centre of the Shortt measurement exactly
matching the current height of the benchmark, representing no sea level
change at all since 1888.

Can we See Sea Level Rise?

Fig.13 - Eaglehawk Neck today

Visual evidence of the coast in and around Port Arthur also suggests
no sea level rise since the 19th century. For example, Commandant
James Boyd, an officer of the colony in 1854 stated that Eaglehawk Neck,
the narrow isthmus of land which allowed the British to control access
to the peninsula, was only 78 yards across [11]. That's about 71 metres.
Today, Eaglehawk Neck is 106 metres wide between the high tide points [12]. Had there been sea level
rise as claimed by Pugh et al., we would expect the neck to be
somewhat narrower
today, certainly not wider.

Lempriere's own tide data from 1841 and 1842 shows that the lowest
tides of all were about 2 ft in his tide gauge, meaning, that the minimum
depth of water was 2 ft if the tide gauge stilling well was sitting on
the sea bottom. If, as is more likely, he had left a clearance of
water below the well to avoid interior silting, that minimum depth would be 3 ft or more. Pugh
et al.
believe Lempriere kept his tide gauge at the Basin next to the stores (see map fig.4). But on a visit there
during low tide, I found the depth of water in the now derelict basin to
be only ankle deep (see photo). Of course it could be silt accumulation even
though much of the sea bottom was stony, with not much water movement
around the cove, but it hardly inspires confidence in the `sea level
rising' claims, and we could find ourselves making endless excuses for each and
every example where sea level rise fails to manifest itself visually.

Old paintings and sketches present the same problem. Water,
water, everywhere during the 19th century, but less water
today. With sea level rise, the opposite should be true.

For copyright reasons, these images cannot be displayed here, but
clicking the following links will bring up these images directly from
the State Library of Tasmania website. (Press the `back' button of
your browser to return to this page).

The last image for 1846 is particularly interesting as it shows a large
3-masted sailing ship moored where the Lawn (fig.14)
now sits.

Here's how the Basin area looks today at low tide. High tide would only
be about 75 cm above what is shown here, or 2½ feet.

Fig.14 - The Port Arthur Basin and Stores area at low tide

Photographs, images and paintings cannot always tell the
full story, but the lack of any visible sign of sea level rise is a
serious problem for the Pugh et al. claims. The `Lawn' area in the
above photo was actually covered by water during the early 19th century,
but it was slowly reclaimed over a period of decades. One man buried
on the Isle of the Dead actually drowned where that lawn now sits. The
Lawn area was not reclaimed because of any shortage of land - there's land in abundance all
over the area, and the lawn was never built on or put to any productive
use. For these reasons, it's not clear why the
authorities even bothered. Perhaps it began to smell as the water got
too shallow and stagnant over time.

Here's one further visual image - the Isle of the Dead
painted in the 1840s by Catherine Augusta Mitchell, wife of the
superintendent of the Point Puer boys prison [13]. Compare
it to the photo I took of the isle from the same angle in fig.5. Was
artistic licence involved here? The words at the bottom of the
painting say -

Fig.15 - An early 1842 painting of the Isle of the Dead by Catherine
Mitchell

This was less a piece of fanciful art, more a tragic
memorial on canvas, in a time before photographs.

Fig.16 - The Pugh and Alternative scenarios compared - with previous
revisions included

From the foregoing, the Pugh et al. study is faced with this problem.
Should the sea level trend line in fig.16 be the red one, or the purple
one? The purple one is that suggested by the authors at the lecture, but is
clearly untenable given that we cannot now justify the excessively long 1888 error bar. Even with the original error bar, it only then barely
squeezed in using the upper extreme of 1841 and the lower extreme of
1888. The red sea level trend line is physically impossible.
There was slight global cooling going on during the 1841-1910 period, and that
would if anything cause a sea level fall of about 3 cm, not a rise. In
addition, the land itself is measured by them to be uplifting very slowly - 3.2
cm over the full 160 years. That means their sea level rise would have
been swimming uphill against global cooling and rising land, a physical
impossibility given the climatic circumstances and the available evidence.
Probability statistics may attempt to explain it away,
but cannot explain it.

Fig.17 - 19th century temperatures at Hobart and Melbourne combined

Since a sea level rise of that magnitude during the 19th century is
simply not credible, particularly as it was cooling during much of that
time as shown in the above graph [14],
we must go back further in time to
question the validity of the authors' original placement of the benchmark in 1841.

In the 20th century, while the IPCC claims a sea level
rise of 10-20 cm globally, that has not occurred around the Australian
coast. According to the National Tidal Facility (NTF) in Adelaide [15], a survey of 27 long-term tide
gauges around the Australian coast (which faces
three oceans) found a rate of sea level rise of only +0.3 mm/yr,
equivalent to a rise over a century of just 3 cm, not 10-20
cm. Even that 3 cm is queried by them because two of the tide gauges
they surveyed were on subsiding land in Adelaide harbour, with no
such subsidence at nearby sites. If those two tide gauges are excluded
from the sample, we end up with a sea level rise of only +0.16 mm/yr,
equivalent to 1.6 cm for a full century. Whether 3 cm or 1.6 cm, the
sea level trend suggested in this report for Post Arthur after 1888 is
consistent with the results of the NTF survey but not consistent with the
conclusions of Pugh et al.

In their paper Pugh et al. drew particular attention to
the 90-year tide gauge record from Fremantle in Western Australia, with a sea level rise rate of +1.38 mm/yr, equivalent to 13.8
cm in a century. That, they felt, vindicated their figures. On
further investigation, it seems Perth (adjacent to
Fremantle, both being one big urban sprawl) is subsiding as indicated
by altimetry satellites. Other tide gauges around the Western Australian
coast do not support the Fremantle picture at all.

The average for all of them, including the errant
Fremantle, is a sea level fall of -0.31 mm/yr.

In other words, the comparison with Fremantle was a red
herring. Comparison was also made with Fort Denison in Sydney, where
a sea level rise of +0.86 mm/yr is indicated by the NTF. However,
the record for that location kept with the PSMSL [18] shows the sea level rise there occurred
almost entirely in the first half of the 20th century, with little change
since. Again another red herring. (For
more background on the Fort Denison record, see the Appendix here).

The only proper basis
for comparison is the average of the whole Australian coast, not selected
locations, and that
Australia-wide average, excluding sinking Adelaide, is +0.16 mm/yr,
or 1.6 cm for the century. This is closely in line with what we see between
Shortt's measurement of 1888 and the sea level at Port Arthur today.

The Big Day, 1st July,
1841

Fig.18 - Ross

Finally, we come back to where it all started -
the day the benchmark was first carved into that cliff 162 years
ago.

It began with Baron Von Humboldt who conceived the idea
of having sea level benchmarks placed at strategic points around
the world in order to ascertain any relative changes in levels
between land and sea. James Clark Ross(fig.18) heard about this idea when he returned to Tasmania from his voyage to
the Antarctic. He even bemoaned the lost opportunity of placing
such marks in Antarctica, since he heard about the idea only
after he got back to Tasmania..

When Ross returned to
England after his long voyages to the Antarctic and southern
seas, he wrote the following account in his book [3] of how and why a benchmark
was necessary -

"The fixing of solid and
well secured marks for the purpose of showing the mean level of the
ocean at a given epoch, was suggested by Baron von Humboldt,
in a letter to Lord Minto, subsequent to the sailing of the
expedition, and of which I did not receive any account until our
return from the Antarctic seas, which is the reason of my not having
established a similar mark on the rocks of Kerguelen Island, or some
part of the shores of Victoria Land. ..." (in the Antarctic-JD)

But all was not lost.

His friend, Governor Sir John Franklin, informed him that an official
of the convict colony at Port Arthur, Thomas Lempriere, had been keeping
tide measurements for some time and that perhaps that would afford a
means of placing a sea level benchmark at Port Arthur. Ross seized
the opportunity and went to Port Arthur, with the Governor, in May 1841
to meet Thomas Lempriere - [17]

"My principal object in
visiting Port Arthur was to afford a comparison of our standard
barometer with that which had been employed for several years by Mr.
Lempriere, the Deputy Assistant Commissary General, in accordance
with my instructions, and also to establish a permanent mark at
the zero point, or general mean level of the sea as
determined by the tidal observations which Mr. Lempriere had conducted
with perseverance and exactness for some time: by which means any
secular variation in the relative level of the land and sea, which is
known to occur on some coasts, might at any future period be detected,
and its amount determined. The point chosen for this purpose was the
perpendicular cliff of the small islet off Point Puer, which, being
near to the tide register, rendered the operation more simple and
exact; the Governor, whom I had accompanied on an official visit
to the settlement, gave directions to afford Mr. Lempriere every
assistance of labourers he required, to have the mark cut deeply in the
rock in the exact spot which his tidal observations indicated as the
mean level of the ocean."(my emphases
- JD).

With such an explicit explanation of what
the benchmark was for, and where it was placed, it becomes perplexing
that Pugh et al. did not take this testimony seriously enough to exhaust
every possible avenue by which their study could avoid contradicting
this very clear account. But contradict it they did - they took
the view that the benchmark did not end up at `zero point' at all, but
at a height closer to high tide.

They justified this view by citing what
Ross himself said a little later in his narrative. Speaking of any
future such sea level mark, he said -

"I may here observe,
that it is not essential that the mark be made exactly at the mean level
of the ocean, indeed it is more desirable that it should be rather above
the reach of the highest tide: it is, however, important that it be made
on some part of a solid cliff, not liable to rapid disintegration, and
the exact distance above the mean level (which may also be marked more
slightly) recorded on a plate of copper, well protected from the
weather..."

He added -

"The most desirable
position for such another mark would be near the north-west extremity of
the island, and in the vicinity of Cape Grim...". (Cape Grim is at the
north-western tip of Tasmania - JD).

What Ross was referring
to here was not the mark on the Isle of the Dead - he wrote about that
in the past tense, as something already done,
`rendered' - but rather about prospective future
marks. These passages can in no way be interpreted as modifying or
negating what he wrote earlier about the Isle of the Dead
benchmark. To impute such a meaning to it is quite
misleading. Besides, the benchmark was not even struck `above the reach of the
highest tide', but rather at some point below high tide and fully exposed to
the sea.

Ross did eventually get
an opportunity to strike a second mark, this time in the Falkland
Islands, and he did exactly what he said above - struck it over 5 feet
above mean sea level, `above the reach of the highest tide', with a plaque
to indicate the exact height of the mark above the real mean sea level.
That particular mark in the Falklands now poses a puzzle of its own - it shows no sea
level rise.

Pugh et al. deny the accuracy of the Ross
testimony.
Are they saying Ross was mistaken? A liar? Did Lempriere do
something contrary to what Ross and the Governor ordered? It should take compelling evidence to contradict such a clear and precise testimony
by someone with the stature of James Clark Ross..

But, let us assume Pugh et al. were right and
that the benchmark was not struck where Ross said it was. That
would mean that (a) Ross was a liar, or (b) Lempriere did the job alone
and struck the mark in a different place, or in a different way, to what
Ross instructed, or (c) Ross had a lapse of memory about it all and
misreported the whole episode in his book.

Taking each in turn,

Scenario (a) is a
non-starter. Ross was a respected officer of the Royal Navy, and a
scientist and explorer. The Ross Sea in Antarctica is named after him. He
knew only too well the importance of exactness when it came to
scientific endeavours. He was also a mariner, and seafarers have
their own set of ethics, one of which is to report navigation
information accurately - lives may depend on it.

Scenario (b) is possible, but only if Ross
was not present at the time the benchmark was struck. Had Ross
been supervising the operation personally, or just overseeing it, he
would not have reported the mark as being at mean sea level.
In addition, Thomas Lempriere had a wife and 11 children in cramped
quarters at Port Arthur. He was desperately seeking both promotion
and better accommodations for his large family, and it is therefore
unthinkable that he would perform his task in any way other than to follow the
instructions of Ross - and indeed the Governor - to the letter.

Scenario (c) is not credible
either. Ross was only 41 years old when he met Lempriere, and his
book was published only 6 years later, making age-related memory loss very
unlikely for such an important project by a man in his prime. His sense of seafarer's ethics would also have persuaded him
to report the positioning benchmark with proper attention to accuracy as
other seafarers might come across the benchmark sometime in the future..

Of the three scenarios, only (b) (i.e. Lempriere acting in defiance of instructions) could
possibly explain the benchmark being struck at a height different to
that reported by Ross. This then raises the question - was Ross present at
Port Arthur on the day? If he was present, then there is no
way the benchmark would have been stuck at near high tide - and then for
Ross to misreport it in his book. If he was present, then we can
be confident the mark really was struck at what Ross and/or Lempriere
estimated MSL to be.

We have no confirmed evidence that Ross
was present on the day. What we do have is four items of
circumstantial evidence -

1) A cruising yachtsman visited the Isle
of the Dead in 1891
[4] and saw the benchmark
there. He also saw something else - a small stone tablet mounted above the benchmark, "...erected it is said by
Captain Ross, of the Erebus and Terror Antarctic
expedition, during the interval between the visits to the south polar
regions." We will return to that stone tablet later,
but since 1841 was within the living memory of many of the local people
of 1891,
their collective memory that Ross struck the benchmark suggests he was
present on the day. Ross was not the kind of man people would forget
easily. He was very famous.

2) Ross was positively placed at Hobart
on 29th
June, 1841 (when he was engaged in compass
calibration) and also on 6th July (when he hosted a farewell soiree for his friends on the
eve of his departure from Tasmania)[3], so there
was nothing to prevent Ross making a brief overnight visit to Port Arthur to
oversee the striking of the benchmark. Had Lempriere
deviated from the original plan in Ross' absence, he had sufficient time
and means to send a message to Ross to that effect, either by the very
quick semaphore signalling system which linked Hobart and Port Arthur,
or by sending a letter by sea, or even by meeting Ross' ships (the Erebus and Terror) as they passed the harbour mouth of Port Arthur a week
later on their way to Sydney. Since Ross published his account
six years later in 1847, there was even ample time for normal mail.

3) Pugh et al. stated this
in their paper - "Ross
may have been present, although preparations for the departure of his
expedition from Hobart on 7 July would have also demanded his
attention. Examination of the copy of the official letter-book
(now in the Scott Polar Research Institute, Cambridge; called `Ross
Family Papers' in Ross, 1982) show that no official letters were signed
by Ross around 1 July." Since the ships were due for departure
from Hobart within a week, the lack of signed letters around that day suggests Ross was
out of town. Where? At Port Arthur? Port Arthur was the only
matter important enough to pull him away from Hobart during that final
week of his visit to Tasmania.

(4) Lempriere kept a
meticulous tide log. He never missed a single tide come rain or
hail or storm over the 2-year period covered by his tide log from
January 1841 to December 1842. (This fact alone suggests he was a
meticulous man who would not capriciously put the benchmark somewhere
other than where the Governor and Ross told him to put it.)

But something strange happened on
1st July. Here is how his log looks both before 1st July and
after.

Fig.19 - The Lempriere tide log between June and July 1841

Look at the sudden change of the formatting. The upper part for
June 1841 shows long dashes between numerals, making it possible for
someone to confuse what number refers to what. Suddenly, as of 1st
July, the formatting changes from a long dash to a mere dot, much
clearer and less prone to reading error. Who advised him to make
that change? The upper formatting was present on all pre-July
logs, while the lower formatting style was present on all subsequent
logs. The abruptness of the change suggests someone asked him to do
it for clarity's sake, and did so on 1st July 1841. That someone could only be Ross as there
was no-one of seniority in Port Arthur who was interested enough in
Lempriere's tide log to suggest or demand such a change. Lempriere is
even on record as wanting to please the scientific community as much as
possible and a `suggestion' by such a luminary as Ross would be all the
excuse Lempriere would need to change the formatting style of his tide
log.

Individually, all these are merely circumstantial. But
collectively, they strongly suggest Ross was present on the day,
and that makes his narrative all the more compelling.

Ross sailed away from
Hobart on his ships Erebus and Terror on 7th July 1841, the benchmark already struck, with or without
his presence on that July day in 1841.

The `Curious Little
Stone'

The mystery does not end there. The striking of the benchmark
left an intriguing postscript, one which set the Pugh et al. team off in
one direction - and this author off in another.

Someone, probably Lempriere, had a small stone tablet mounted
near the
benchmark to explain its purpose. The Isle of the Dead had numerous gravestones, but this one was quite different as it sought to explain
what the benchmark was all about. The tablet went missing early in
the 20th century so we have no way to confirm today what was written on
it. But we do have the testimony of two people who transcribed its
words. The first was our Mr. Mason who not only
measured the height of the benchmark in 1888, but also told Cmdr. Shortt
what was written on it. Here's what he transcribed -

"On the rock
fronting this stone a line denoting the height of the tide now struck on
the 1st July,
1841, mean time, 4h. 44m.
p.m.; moon's age, 12 days; height of water
in tide gauge 6 ft. 1 in."

But, three years later in
1891, we have this account by the visiting cruising yachtsman who
sketched and accurately transcribed the words from the gravestone of one Joseph
Kerr. Then he noticed -

"A few yards
away, on the rocks fringing the shore and facing the east, was a
curious little stone, erected it is said, by Captain Ross, of the `Erebus' and
`Terror' Antarctic expedition, during the interval
between the visits to the south polar regions. It bears the following
inscription:-

"On the rock
fronting this stone a line, denoting the height of the tide, was struck on the 1st
July, 1841. Mean time, 2.44 p.m. Moon's age, 12 days. Height of water in the tide gauge,
6 ft 1 in."[3]

The two accounts differ from each other in the text coloured bold brown. We have a clear conflict over the time of the
striking of the benchmark (2.44 p.m. versus 4.44 p.m.), and a grammatical
difference - `now struck' versus `was struck'.

But there are subtler differences. Notice the formatting of the
times, Mason's clumsy `4h. 44m. p.m.' versus the yachtsman's simpler `2.44 p.m.'
Now look at fig.19 above at Lempriere's log entries. Lempriere uses
the exact same formatting style
for times (post-1st July) as the yachtsman used, but not Mr.
Mason. There are subtler differences still, such as the
yachtsman's use of capital letters at the start of sentences, and the
yachtsman's inclusion of commas.

Fig.20 - Joseph Kerr gravestone

Which version is the more
credible? The yachtsman's visit to the Isle of the Dead
did not stop merely with the tablet. He also looked at
some of the gravestones, one of which he recited word for word
in his article in The Australasian magazine, and even
drew an accurate sketch of it.

That gravestone still exists today and is fully readable in
spite of the passage of time. It is the gravestone of one Joseph
Kerr, and the yachtsman's version of what was written on it
matches the wording on the same gravestone today exactly,
word for word.

It is for this reason that the yachtsman's version of what
was written on that `curious little stone' should be treated
more with much more credibility than it has been.

Furthermore, Cmdr. Shortt added this to Mason's reading of
the tablet -

"It is stated by my
informant, Mr. Mason, that the words and figures underlined
are nearly obliterated, and that he has given what they appear
to be."

Looking to Mason's version above, we see the phrase `now struck'
underlined, as per Shortt's qualifier, but there were no figures underlined as Shortt claimed. We are therefore
left to ponder exactly which figure or set of figures were problematic. The
time? The tide height?

The yachtsman gave no indication of having any problem with reading the
words on the tablet, so we are left with something of a puzzle
here. Which time is the right one? Which tide height
applies?

Taking them in turn, the time of 4.44 p.m. reported by Mason is
almost consistent with a height of 6ft 1in in Lempriere's tide gauge, based on the data in his log, but 2.44 p.m. definitely is not.
However, 4.44 p.m. (4.55 p.m. modern Australian
Eastern Standard Time) is a very strange time of day to be doing important
survey work - close to the winter solstice of the southern hemisphere,
virtually at twilight, and close to high tide when striking a benchmark
would be both difficult and potentially dangerous due to the depth and turbulence of water at the
cliff face. On the other hand, the yachtsman's 2.44
p.m. in mid afternoon makes much better sense as there was good light
and a lower tide.

Fig.21 - Blow-up of Lempriere's tide log

But both agree on 6 ft 1 in.
Does that not settle the issue? Not exactly. The
yachtsman's time of 2.44 p.m. is more consistent with 5 ft 1 in in
Lempriere's tide gauge, not 6 ft 1in.

Now look at this blow-up of Lempriere's tide log and examine
the 5's and 6's. The fourth one down on the left is
particularly confusing, as is the third,

Even in our modern era, it is common for `6' and `5' to be
confused when looking at photocopies, faxes, or other
imaging. But what of a corroded 50-year-old `curious
little stone' sitting on a cliff exposed to the wind and the
waves?

There is a distinct possibility that both men got the tide
height wrong due to that 6-5 confusion, especially given Mason's
remarks about the tablet being eroded and difficult to read.

Fig.22 - A `6-5' gravestone

Here is another gravestone on the
Isle of the Dead. It
has a `6' and a `5' adjacent to each other in the lower right of
the stone. It is in good condition because it is in a sheltered
position.

Give it 50 years of erosion from wave
action, let a little chip fall out of the
left side of the `5' and you have an instant `6', It's as
easy as that.

Pugh et al. went 100% with Mr. Mason's 6ft 1in/4.44 pm
interpretation, placing the benchmark a foot higher (30.5 cm) than
the height which would match the yachtsman's 2.44 pm time, and
in complete denial of the Ross narrative.

That choice locked them into (a) denying the truth of the
Ross narrative, (b) incurring a 13 cm sea level rise in spite of
the 1888 Shortt observation and the NTF survey which would suggest no such rise,
and (c)
denying the validity of the yachtsman's observation of the time in spite of
the fact that he got the Joseph Kerr gravestone exactly right.

Pugh et al. did address some of these
problematic issues in their paper and recent lecture including the Ross
narrative. However, at a previous lecture on 5th April 2000 given to the Royal Society of
Tasmania by the same authors, they did not mention the Ross narrative or the yachtsman's reading at all.
On that occasion, they simply reported on Mr. Mason's 1888 reading and
proceeded from there.
The mostly scientific audience remained unaware that additional
significant evidence existed, evidence which did not favour the scenario
of sea level rise being presented.

In the end, I raised the issue of the Ross narrative and the
yachtsman during question time.

[6]
Lempriere, T. (1841-1842) "Register of
Tides Kept at Port Arthur", from the
Royal Society archives, London

[7] Shortt,
Cmdr. J., (1889), "Notes on the
Possible Oscillation of Levels of Land and Sea in Tasmania During
Recent
Years", Royal Society of Tasmania papers,
State Library of Tasmania, Hobart, pp.18-20.

[8] Shortt,
Cmdr. J., (1885), "Summary
of Observations on Earthquake Phenomena Made in Tasmania During 1883
and
1884", Royal
Society of Tasmania papers, State Library of Tasmania, Launceston,
pp.263-270.

Fort Denison which lies in the outer Sydney Harbour, New South Wales,
Australia. Mean Sea Level is presented. The graph below
shows monthly sea level since 1914 smoothed with a 12-monthly
running mean to reduce monthly `noise'. Fort Denison with 82 years
of data shows a sea level rise of +0.86 mm/yr (or
8.6 cm if taken over a full century). Data from the Permanent
Service for Mean Sea Level.

In a recent paper on the
Isle of the Dead sea level benchmark at Port Arthur, Tasmania, struck in
1841 by Ross and Lempriere, Hunter et al (GRL, v.30, No. 7, 1401, April 2003)
claimed that sea level had risen there by around 1 mm per year (or 10 cm over a full century), citing a
comparison with Fort Denison and Fremantle in Western Australia as cross
validation for their statistical study.

According to Australia's National
Tidal Facility, Fort Denison is only one of three tide gauges around
the Australian coast to have more than 60 years of historic sea level
data. The other two are Port Pirie in South Australia and
Fremantle in Western Australia. Fremantle is the oldest (90 years), but because recent satellite
measurements have shown Perth (near Fremantle)
to be subsiding steadily, its 90-year tide gauge record cannot be used
in sea level studies because the land subsidence creates an artificial
sea level `rise'. Other tide gauges on the same coast as Fremantle
show an average sea level fall over the last 30 years , further
invalidating the Fremantle record.

The average for all of
them, including the errant Fremantle, is a sea level fall of -0.31 mm/yr.

With Fremantle clearly
invalid, that leaves only Fort Denison and Port Pirie of the
three 60-year plus stations.

On the face of it, Fort
Denison's sea level rise would seem to support Hunter et al.
But does it?

A careful look at the
plot of Fort Denison below shows that most of the sea level rise there
took place in a 3-year period between 1948 and 1951 when an apparent
stepwise rise in long-term sea level, of about 6 centimetres,
occurred. This is impossible physically. Sea level cannot
simply rise 6 cm in three years from one long-term level to another
higher level. It is also unlikely to be caused by land subsidence
as that too is a slow cumulative process as happens at Fremantle, not a
sudden step action. Earthquakes or tremors could cause it - if
severe or numerous enough - (such as the 1,000
tremors which happened in eastern Tasmania during the early 1880s, and
which may have affected sea level at the Isle of the Dead during that
period.).
But that seems very unlikely in the case of Sydney Harbour. That
leaves instrumentation problems as the only viable explanation for the
sudden datum shift around 1948-51. Such datum shifts caused by
instrument problems are very common with tide gauges, such as those of
Hobart shown here. Simple fouling
by sea weed, marine growths and debris is all it takes to skew readings
from a tide gauge. Relocation or instrument replacement can also
cause such shifts.

But suppose Hunter et
al. was right and that the Fort Denison datum shift was a genuine
sea level rise and not an artifact of instrument problems. Such a
large sea level rise could not go un-noticed at the other long-term tide
gauge in Australia - Port Pirie in South Australia

The
Port Pirie record for 1945 to 1954 is shown left, with the vital
3-year period when Fort Denison recorded that big `sea level
rise', shown by the thick green line.

Not only is there no sea level
rise in that 3-year period, the adjacent years show no sea level
rise either.

In fact, the NTF says that the
tide gauge at Port Pirie has shown a sea level fall of -0.19 mm/yr over its full 63 years of
record. With both the Fort Denison and Fremantle sea level
`rises' clearly invalid, this leaves the Hunter et al
claim about a similar sea level rise at Port Arthur, Tasmania,
in a lot of difficulty.